1. Field of the Invention
This invention relates to the precision counting of electrons, ions and other charged particles, as required for physical measurements of surfaces and in many other fields. It particularly relates to a method and apparatus for counting charged particles generated in a short time of within several dozen nanoseconds.
2. Prior Art Statement
FIGS. 1 to 3 show the configuration of a conventional apparatus for measuring the number (intensity) of charged particles. FIG. 1 is drawing of a conventional apparatus used for measuring large numbers of continuously incident charged particles. Charged particles 1 are collected by a current collector 2 and an ammeter 3 is used to measure the charges carried by the charged particles 1. Among the advantages of this method are that the measuring system is simple and offers good reliability when there are large numbers of incident charged particles.
For the counting of small numbers of incident particles and when the current regions are so small as to be approaching the limits of current measurement, as shown in FIG. 2, the system is provided with charge multiplication gain functions such as constituted by a electron multiplier 4 or the like arranged as the front end of a current collector 2, and power from a power source 5 is applied to the electron multiplier 4 to increase the total current of the charged particles incident on the electron multiplier 4 and to carry out the counting by means of an ammeter 3.
An advantage of this method is that for each charged particle 1 incident on the cathode of the electron multiplier 4 a multiplicity of secondary electrons are emitted, ultimately enabling around 10.sup.6 to 10.sup.8 electrons to be generated, so that even when the number of incident particles is relatively low (in the picoampere range) the signal level can be increased to enable current measurement to be carried out.
In contrast to these current measuring methods in which total collector current is measured, in the apparatus shown in FIG. 3, a charged pulse amplifier 6 and a pulse counting unit 7 are provided in the following stage of the current collector 2 and the number of pulses generated each time a charged particle 1 enters the electron multiplier 4 is counted using a pulse counting unit (pulse counting method).
Among the advantages of this method are that it can be applied to absolute counts of incident particles and it offers a good level of counting reliability even when the numbers of incident particles are low.
However, each of the above conventional methods have drawbacks, as described below. With the arrangement of FIG. 1, when the number of incident particles is low or in the case of incident particle pulses, owing to factors including the response characteristics of available ammeters, it is impossible to obtain a correct measurement of the number of particles in terms of the amount of current. For example, with generally available ammeters, even highly sensitive ones, the current intensity is limited to around 10.sup.-12 (in terms of numbers of incident particles, this is equivalent to 10.sup.7 /sec).
With the arrangement of FIG. 2, while the overall signal level is increased, easing the current measurement conditions, owing to the fact that the absolute value of the multiplication gain of the electron multiplier 4 used to detect the charged particles is not known the absolute number of incident charged particles cannot be found. Furthermore, since in the course of the multiplication gain process it is statistically impossible to avoid some fluctuation in the multiplication factor of the electron multiplier 4, the multiplication gain varies from particle to particle, so that even when the numbers of particles incident on the electron multiplier 4 are the same, there is a risk that they will be measured as a different output current value. Moreover, if this method is used to count charges when the particles are generated as pulses, the response characteristics of the ammeter connected downstream of the electron multiplier remains a problem.
Also, while numbers of particles counted with the arrangement shown in FIG. 3 can be made to correspond to absolute numbers of incident particles, the dead time of the electron multiplier and the pulse counting system response characteristics limit count ratios to, at most, a few particles per several tens of nanoseconds, and larger numbers of incident particles that will give rise to a counting loss.
Thus, there are major drawbacks with each of the above conventional methods which make it impossible to achieve particle counts with high reliability when the numbers of particles generated are relatively low, meaning from several to several hundred particles, and the generation takes place in an extremely short time, such as the several tens of nanoseconds in which ionized particles are generated by a pulsed laser beam.